Deeper and more difficult reservoirs are demanding different concepts offshore West Africa, including tension leg platforms. Jérôme Iacovella and Pauline Beraud discuss the options.
|The Moho Nord TLP design. Image from Doris Engineering/F. Lucazeau.|
The wellhead platform with tender assisted drilling (TAD) has been the preferred concept for many developments in the benign environmental conditions found in West Africa and southeast Asia for many decades.
Replacing the fixed jacket used in shallow water with a tension leg platform (TLP) allows the wellhead platform concept to be extended into deeper waters.
The first TLP installed in about 150m water depth on the Hutton Field in the North Sea in the early 1980s has now been decommissioned. The second, installed five years later in the Gulf of Mexico (GOM), was the Jolliet tension leg wellhead platform (TLWP). It was four times smaller than Hutton, with a displacement of only 16,600-tonne, and is still producing.
Since then, 15 TLPs have been installed in the GOM and five TLWPs have been installed in southeast Asia and West Africa, mostly with tender assisted drilling. Two TLWPs that will use semitender assisted drilling (STAD) are currently under construction: the Papa-Terra TLWP for Petrobras in Brazil and the Moho Nord TLWP for Total in the Congo.
A TLP is a floating platform, vertically moored by means of tendons and anchored with driven piles. The tendons are typically steel pipes, which are kept in tension at all times by the buoyancy of the hull. The structure is vertically restrained by the tendons and the high axial stiffness virtually eliminates all the vertical heave, pitch and roll motions induced by the environmental conditions. A TLP has a very limited storage capacity and as a consequence can only be used in areas where the production can be exported directly through a pipeline or via a storage unit, such as a floating storage and offloading vessel or a floating production, storage, and offloading (FPSO) vessel.
In West Africa, oil storage and offloading is generally required, which means that an FPSO is the obvious choice for a host platform. The only exceptions are where an export pipeline is installed and a compliant piled tower (CPT) or smaller FPU barge without storage can be used. With a host platform, the two main field development alternatives are then:
- Subsea wells tied back to the host platform through flowlines with umbilicals.
- Surface wells on a dry tree wellhead platform installed close to the host platform with fluid transfer lines (FTLs) and minimum processing facilities.
For the shallow depth Oligocene and Miocene reservoirs found in West Africa, the subsea tieback is often the more economical solution. In the deeper and more difficult reservoirs, where the wells can be drilled from a single drill center, a wellhead platform offers several advantages that can make this the most economical concept.
First, direct access to the wells facilitates well intervention operations and gives better control over the production for reservoir management, which can increase the oil recovery rate. Second, drilling operations can be done directly from the platform with a dedicated drilling/ workover rig or with a self-erecting derrick equipment set (DES) installed and supplied by a tender vessel.
When the surface wellhead platform with dry trees shows an advantage over the subsea trees then there is a choice between three concepts; the deep draft semi, the SPAR and the TLP. In West Africa, the long period swells have an adverse effect on the dynamic behavior of the deep draft floaters, like the semi and the SPAR, and the TLP becomes an obvious choice. The only drawback with a TLP is that it is designed for a given payload, which implies a good evaluation of the weight from the start, including any future modifications, and accurate weight control throughout the project.
Doris Engineering has been developing the TLWP concept for more than two decades, analyzing pros and cons, adapting technology, and proposing innovative solutions for the particular West African environmental conditions found in the Gulf of Guinea.
Exxon Mobil showed the way when they installed the first two TLPs in West Africa. The Kizomba A TLWP was installed in 2004, on Angola’s block 15, 300m from the host FPSO. The key parameter for this concept development selection seemed to be the need to develop the early-Aptian reservoir and the requirement for careful reservoir management with frequent well interventions. The TLWP hosted the production dry trees and subsea wells were dedicated to gas and water injection. The same development scheme was retained for Kizomba B, which followed in 2005.
The Moho Bilondo development
The Moho Bilondo field, offshore Congo, Moho Nord TLWP for Total in the Congo.A TLP is a floating platform, vertically moored by means of tendons and anchored with driven piles. The tendons are typically steel pipes, which are kept in tension at all times by the buoyancy of the hull. The structure is vertically restrained by the tendons and the high axial stiffness virtually eliminates all the vertical heave, pitch and roll motions induced by the environmental conditions. A TLP has a very limited storage capacity and as a consequence can only be used in areas where the production can be exported directly through a pipeline or via a storage unit, such as a floating storage and offloading vessel or a floating production, storage, and offloading (FPSO) vessel. In West Africa, oil storage and offloading is generally required, which means that an FPSO is the obvious choice for a host platform. The only exceptions are where an export pipeline is installed and a compliant piled tower (CPT) or smaller FPU barge without storage can be used. With a host platform, the two main field development alternatives are then: has been developed in two phases. In 2004, Doris Engineering performed a conceptual study of a TLWP with tender assisted drilling for the Moho Bilondo development that was compared to a solution with two subsea drill centers tied back directly to the FPU barge. On this occasion, the subsea solution was selected and the FPU was installed and brought on-stream in 2008. Nevertheless, this early work proved the feasibility of a number of key technical issues and laid the foundations for the future development.
|Hutton TL legs-the first TLP installed-at Scotland's Cromarty First, ost field decommissioning.|
In 2011, for the Moho Nord development, Total again included the TLWP in the concept screening process when looking at how to produce the Albian reservoirs, which are deep formations more than 3000m below mudline. The complexity of these reservoirs imposed a high level of reservoir management with a need for frequent well intervention. It was this requirement that led the project team to select the TLWP concept, with dry trees that would allow drilling and coiled tubing operations to be performed simultaneously.
Doris Engineering has been involved with this project since the conceptual and the pre-project studies and subsequently as the engineering subcontractor of Hyundai Heavy Industries, which as the successful bidder of the compensated call for tender (CCFT). During these engineering studies, the following new features have been introduced by Total to make the Moho Nord a unique TLWP concept:
- Semi tender assisted drilling (STAD) unit is positioned on the east side of the TLP and the derrick equipment set (DES) sits on skid beams that run in the North South direction to allow the coiled tubing (CT) to be run underneath the Substructure.
- The DES substructure has a clear height of 13m and the main BOP is placed below the upper deck to allow enough clear space for a stand-alone coiled tubing tower to operate independently without interfering with the drilling operations.
- The drilling riser system has been designed to jump from one well to another without being pulled out of the water. The drilling riser is hung off at the mezzanine deck in a slot with a skidding system and is only disconnected from the wellhead at the seabed when moving to the next target.
These innovations combined can provide a significant saving in the drilling durations with an associated saving on the overall drilling costs.
In the benign environmental conditions found in West Africa and southeast Asia, the TLWP with tender assisted drilling (TAD) has been found to be an interesting solution in deep waters. It is a robust concept when dry trees are required to improve reservoir management and facilitate well intervention operations and with the recent innovations in the drilling and well intervention systems it may become the cost effective solution for other similar developments in the future.
Jérôme Iacovella joined Doris Engineering in 2000 as a naval architect. He is now heading the naval architecture and marine operations department. He has been particularly involved in the development of Moho Nord TLP project for Total.
Pauline Beraud joined Doris Engineering as anaval architect in 2006. For more than three years she has been involved in the development of Moho Nord TLP project. She holds and engineering degree from ENSTA Paris Tech.